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how many sprinkler heads per zone?

depends on how many Gallons a minute you have available and what the precipitation rate of the sprinklers are and how much loss there is in the system.

70 psi will not be 70 psi at the sprinkler heads, you must calculate the pressure loss in the pipes. Loss depends how big and how long the pipes are, the valves you are using, backflow you are using, loss of the meter, etc....

What type of heads are you using and what nozzles are in them? The flow rate of the nozzles is given from the manufacturer in a chart based on pressure to the heads. Generally speaking, the flow is limited by the smallest piece of pipe in the system to the valve. If you use 1" main and zone lines, but only have a 5/8th's" meter, you are limited by the meter. That will set your available flow to the house and system. In the area I'm in, a 5/8th's" meter with 70 psi will give me enough water to run 6 or 7 heads, depending on nozzle selection. To make sure you know your flow, use a head with a pressure regulator in it and it will give you a fixed pressure out the nozzle. This will allow you to make very exact calculations. Add your flows up and see where you are at.